Harmonic separating filters



Sept. 22, 1970 e. LUZZATTO 3,530,405

HARMONIC SEPARATING FILTERS Filed Jan. 6, 1969 TRANSMITTER {Vim A United States Patent 3,530,405 Patented Sept. 22, 1970 US. Cl. 333-6 7 Claims ABSTRACT OF THE DISCLOSURE A harmonic separating filter has at least two input line stubs one of which includes a section of line of one-eighth of the wavelength at the fundamental frequency; a main useful load to be fed through the one-eighth wavelength section with energy at the fundamental frequency from a radio frequency transmitter; and a quarter wavelength line section connected in shunt with the useful main load (when connected). The other of the line stubs feeds energy at the second harmonic frequency to an auxiliary load when connected to a nodal point at the centre point between two line sections each of one-quarter wavelength and together constituting the other line stub.

vided that it is periodic and can be resolved into component sinusoidal waves at different frequencies.

The main object of the invention is to provide improved filters capable of distributing among different load circuits the fundamental frequency and the second harmonic component of an input periodic wave either to enable separate use of said second harmonic component to be made or to enable the amplitude of said second harmonic component to be reduced by absorption.

Another important object of the invention is to provide improved harmonic filters which shall be such that (in contradistinction to known filters using elements with concentrated or lumped constants) the input impedance shall always be matched to the impedance of a source providing the wave input and which shall also be such as to allow for absorption or reflection of even harmonics.

According to this invention a harmonic separating filter includes at least two input line stubs one of which includes a section of line of one-eighth of the wavelength at the fundamental frequency; means for connecting a main useful load to be fed through said one-eighth wavelength section with energy at the fundamental frequency; and a quarter wavelength line section connected in shunt with said useful main load (when connected); the other of said line stubs being arranged to supply energy at the second harmonic frequency to an auxiliary load when connected to a nodal point at the centre point between two line sections each of one-quarter wavelength and together constituting said other line stub.

The invention is illustrated in and further explained in connection with the accompanying drawings in which:

FIG. 1 is an electrical diagram of one form of filter embodying the invention;

FIG. 2 is a cross-sectional view showing one form of mechanical construction of a filter embodying the invention;

FIG. 3 is an electrical diagram illustrating a modified form of filter embodying the invention, and

FIG. 4 illustrates a further embodiment of the invention.

Referring to FIG. 1, the block 1 represents a source of complex waves which are periodic and capable of resolution into a series of component sinusoidal waves. The source 1 could be, for example, a radio-frequency transmitter.

Output from the transmitter or other source 1 is fed to the input of the filter which is the part of the figure within the broken line rectangle 2. The resistance 3 represents a utilisation load which is (in the present eX- ample) assumed to be required to be fed with the fundamental frequency from the source 1.

The filter includes two line stubs 4 and 5 connected at their adjacent ends to an input nodal point N. The stub 4 includes a section of line 6 which is one-eighth of the fundamental wavelength long. This section is the section 6.

In the description which follows, the length of the wave at the fundamental frequency will be represented by appropriate integral fractions of this wavelength.

Stub 6 of length M8 terminates in useful load 3, and this load is shunted by a section of line 7 on length M4 connected at nodal point N The second stub 5 is open ended and comprises two sections of line 8 and 9, each of which is of length M4.

At a further nodal point N situated where the sections 8 and '9 adjoin, is connected a further load hereinafter referred to as the auxiliary load, and in which is employed either to dissipate or to use the second harmonic M2 of the fundamental wave.

With this filter the fundamental frequency present at input node N travels along the M8 section '6 to reach the main load 3; the line stub 5 acts as equivalent to a M2 section; and the M4 line section acts at the equivalent of an open circuit for the wavelength A.

The second harmonic (M2) component at N is unable to reach the main load 3 because it is shunted at N by the section 7 which is for that component wave (M2) the equivalent of a half wave (M2) stub, i.e. is an effective short circuit for the second harmonic.

This second harmonic Wave appearing at N is hoW ever, fed through the section 8 equivalent to M2 and appears at the auxiliary load 10, thus matched, while the line section 9, in parallel with the auxiliary load 10, is also equivalent at the second harmonic frequency to a M2 section and therefore appears as open and therefore passive.

Accordingly the fundamental wavelength x is fed to useful load 3, and the second harmonic wavelength M2. is fed to the auxiliary load 10.

In a similar way odd harmonics are fed to the main load 3 whilst higher even harmonics which are not even multiples of M2 (e.g. the sixth and tenth harmonic) are fed to the auxiliary load 10. Higher even harmonics which are multiples of one-half wavelength (eg the fourth and eighth harmonics) are, on the other hand, reflected since for these the line sections 7, 8 and 9 are equivalent to a section which is one wavelength long or an integral multiple thereof.

FIG. 2 shows a practical form for a filter as represented diagrammatically in FIG. 1. As will be seen it is of the co-axial type of construction. It includes a cylindrical outer member 11, a co-axial conductor 12 which comprises the two line stubs 4, 5 and 7 of FIG. 1 and is made up of the lengths shown in terms ofwavelength A.

The outer member 11 is provided with a first (input) co-axial connector 13 for connection to the source 1 (FIG. 1), a second co-axial connector 14 for connection to the main load 3 (FIG. 1) and a third co-axial connector 15 for connection to the auxiliary load 10 (FIG. 1).

The co-axial connectors are so arranged that they define in the inner conductor 12 the sections of line 6 of M8, between connectors 13 and 14, 8 of M4, between connectors 13 and 15, and two further sections 7 and 9,

each of M4 which are, respectively, effectively short circuits and open.

FIG. 3 shows schematically an adjustable embodiment which can be adjusted to provide matching at different values of fundamental wavelength.

To achieve this result the connectors are in the form of slides 13, 14' and 15 that can be moved along conductor 12 to vary the lengths of the sections of line in terms of fractions of A. The earth connection is also slidably adjustable as indicated. FIG. 4 shows an embodiment in which the sections of line are of the type commonly known as strip lines.

Each of the line sections preferably comprises two or more selectable portions of line, for example as shown in A, B and C, of different lengths so as to enable the filter to be adapted to different fundamental wavelengths.

Selection of the line sections appropriate to a particular value of )t is effected by means of connectors 13", 14" and 15", to which the selected portions of line may be connected at will by means of removable bridges P.

I claim:

1. A harmonic separating filter including at least two input line stubs one of which includes a section of line of one-eighth of a wavelength at the fundamental frequency; means suitable for connecting a main useful load to be fed through said one-eighth wavelength section with energy at the fundamental frequency; and a quarter wavelength line section at the fundamental frequency connected to said one-eighth wavelength section whereby the useful main load may be shunted; means suitable for connecting an auxiliary load to be fed energy at the second harmonic frequency of said fundamental frequency connected to a nodal point at the centre point between two line sections each of one-quarter wavelength at the fundamental frequency and together constituting the other line stub.

2. A filter as claimed in claim 1 including said main useful load and said auxiliary load; said main useful load being connected to said means suitable for connecting a main useful load and said auxiliary load being connected to said means suitable for connecting an auxiliary load.

3. A filter as claimed in claim 1 wherein the first said quarter wavelength line section is connected atone end to a nodal point at the output end of the one-eighth wavelength line section.

4. A filter as claimed in claim 3 and comprising a cylindrical outer conductor Within which is a co-axial inner conductor providing the two line stubs, said outer conductor being provided with an input co-axial connector to a source of input waves and output co-axial connectors for connection to the main useful load and to an auxiliary load, said connectors being so arranged as to divide the inner conductor in effect into the one-eighth Wavelength section and the three-quarter wavelength line sections.

5. A filter as claimed in claim 4 wherein the co-axial connectors are adjustable in position along the conductor 12 in order to enable the filter to be adapted to different fundamental frequencies.

6. A filter as claimed in claim 3 wherein the line sections are in the form of strip line elements.

7. A filter as claimed in claim 6 wherein there is provided for each line section a plurality of selectable strip line elements of different predetermined lengths appropriate to different fundamental frequencies, said strip line elements being selectively connectable in circuit with associated connectors by means of movable connection members.

References Cited UNITED STATES PATENTS 2,421,033 5/1947 Mason 333-6 2,426,633 9/1947 Mason 333-6 2,473,448 6/1949 Rieke 333-76 X 2,661,424 12/1953 Goldstein 333-6 X PAUL L. GENSLER, Primary Examiner US. Cl. X.R. 333 73, 76 

